1. Field of the Invention
The present invention relates to a method of and a system for detecting moisture in a honeycomb panel such as an aircraft body, for instance.
2. Description of the Prior Art
In a honeycomb panel such as wings of an airplane, there exists a problem in that moisture included in the air enters the honeycomb panel due to repeated flights or due to change in atmospheric pressure, and further dew condensation occurs locally therewithin. The moisture in the honeycomb panel will be frozen when the airplane makes a flight at a high altitude, and bonded portions between the honeycomb panel and an outer plate are peeled off due to volumetric expansion of moisture at freezing, thus causing the honeycomb panel to be damaged when the local disbond propagates. As used in this specification and in the claims, a normal portion of a honeycomb panel is a portion containing no moisture and an abnormal portion is a portion containing moisture.
To overcome the above-mentioned problem conventionally, various techniques such as radiographical inspection by X rays, non-destructive test by ultrasonic waves, etc. have been so far adopted.
In addition, as one of the techniques of solving the above-mentioned problem, such a method of detecting moisture in an airplane honeycomb structure has been proposed that infrared images of an airplane structure and onboard appliances are taken with the use of an infrared image forming apparatus in order to inspect the normal and abnormal conditions of the airplane body and the onboard appliances on the basis of the obtained infrared images, as disclosed in Transcript of Lectures (1991) of 29th Airplane Symposium (Oct. 8, 1991) in Japan.
In this moisture inspection method, abnormal portions of the honeycomb panel are detected as follows: still images of the surfaces of the honeycomb panel are previously taken by use of the infrared image forming apparatus; integration images (summation images) are obtained by taking,a plurality of infrared images after the airplane has landed and thereby the temperature of the airplane rises; and the integration images are compared with the still images to detect abnormal portions of the honeycomb panel. Further, in the above-mentioned Symposium, a technique of taking infrared images of an object to be inspected after instantaneous heating and cooling has been also proposed.
Further, another non-destructive inspection method for a bonded structure has been disclosed in Japanese Published Unexamined Patent Application Nos. 3-24449 and 3-82944, in which a structure formed by bonding thin plates is heated up to a predetermined temperature, and the surface temperature distribution during cooling process is measured on the basis of an infrared image in order to discriminate defective positions of the bonded portions.
The above-mentioned prior art methods of detecting moisture in a honeycomb panel involve various drawbacks as follows: In the case of the non-destructive inspection by the radiographical inspection (X rays), a radioactive ray source is required and therefore the handling is not safe; a specific film is required for inspection and therefore the inspection is costly; the workability of moisture detection is low and further the automatization of the moisture detecting method is rather difficult to realize; and it is impossible to detect such a small amount of moisture if each cell of the honeycomb panel is not perfectly filled with moisture, etc. In more detail, in the radiographical measurement based upon a difference in film density for instance, where a plurality of cell samples of a honeycomb panel are prepared and further cells including water are photographed by X rays, a difference in film density between the normal cells including no water and the abnormal cells filled with water is relatively large and therefore distinguishable. However, a difference in film density between the normal cells including no water and the abnormal cells having water partially is relatively small and therefore is difficult to be distinguished.
In the case of the non-destructive inspection by ultrasonic waves, an inspector skilled in the measurement is required; the workability of moisture detection is low; the automatization of moisture detection is difficult; it is difficult to detect a small amount of moisture, etc.
In the case of the moisture inspection due to infrared images, since still images must be previously photographed, it is rather difficult to apply this method to various honeycomb panels other than a specific structure. In addition, since the infrared images are taken when the temperature of the airplane rises after landing, it is difficult to detect a small amount of moisture. This is because a difference in temperature between normal and abnormal portions is small and additionally the detection is subjected to the influence of noise, so that the detection precision is deteriorated, with the result that the resolution power of temperature detection is as low as about 0.3.degree. to 0.5.degree. C.